1. Field of the Invention
The present invention relates to a radiation imaging device that images a radiation image expressed by radiation that has passed through an imaging subject, to a non-transitory computer readable medium that stores a program for controlling the radiation imaging device, and to a pixel value acquisition method of the radiation imaging device.
2. Description of the Related Art
Recently, radiation detectors such as flat panel detectors (FPD) and the like have been realized. In an FPD, a radiation-sensitive layer is disposed on a thin film transistor (TFT) active matrix substrate. The FPD is capable of converting radiation directly to digital data. A radiation imaging device, such as an electronic cassette or the like, that uses this radiation detector to image radiation images expressed by irradiated radiation has been realized. A system for converting radiation to electronic signals in the radiation detector may be an indirect conversion system that first converts the radiation to light using a scintillator and then converts the light to electric charges with photodiodes, or a direct conversion system that converts the radiation to charges in a semiconductor layer containing amorphous selenium or suchlike, or the like. Whatever the system, there are a variety of materials that may be used in a semiconductor layer.
When a radiation image is imaged using a radiation detector, excellent image quality must be assured even while a radiation amount of radiation that is irradiated at an imaging subject is minimized. To acquire a radiation image with excellent image quality, exposure control conditions at a radiation source must be set such that radiation is exposed in a radiation amount that is suitable for an imaging target location. Accordingly, a radiation imaging system has been proposed in which a radiation detector is equipped with an automatic exposure control (AEC) function that detects aggregated radiation amounts of radiation being irradiated thereon after passing through an imaging subject, and that controls a stop timing of the irradiation of radiation from the radiation source on the basis of the detection results. To implement this automatic exposure control (AEC), it has been proposed that pixels for detecting aggregated radiation amounts of irradiated radiation be embedded in the radiation detector, in addition to pixels that are for imaging radiation images.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2012-15913 recites a radiation imaging device in which plural pixels, including pixels for radiation image imaging and pixels for radiation amount detection, are arranged in the form of a matrix in a detection region in which radiation is to be detected, and radiation amounts of irradiated radiation are detected by charges that flow through signal lines connected to the pixels for radiation amount detection being detected.
In a radiation detector with radiation detection pixels for detecting radiation amounts of irradiated radiation, as recited in the above described JP-A No. 2012-15913, if there are defects among the radiation amount detection pixels, it may not be possible to obtain normal pixel values from the defective pixels. If automatic exposure control (AEC) is conducted on the basis of pixel values from these defective pixels, it may not be possible to stop the exposure of radiation at an appropriate timing. Hence, the radiation amount of radiation irradiated at an imaging subject may be excessive or the radiation amount may be insufficient and it may not be possible to obtain a suitable radiation image. Therefore, a process can be considered of: detecting defects occurring among the radiation amount detection pixels in advance; creating and storing a defect map; when actually conducting automatic exposure control (AEC), identifying defective pixels by referring to the defect map; and applying processing to exclude or correct pixel values of the identified defective pixels, or the like.
However, accurately detecting defects at radiation amount detection pixels is not easy in the configuration of a radiation detector (FPD). Namely, in a configuration in which signal lines are shared as transmission paths of charges generated at the radiation amount detection pixels and at the imaging pixels, pixel values of the radiation amount detection pixels and imaging pixels must be separated, and the pixel values of the radiation amount detection pixels alone must be extracted. Further, in a configuration that uses devices such as charge amplifiers and the like for converting the charges generated at pixels to electronic signals, offset components of the devices are superimposed on the pixel values, and there may be cases in which accurate pixel values cannot be obtained. Further still, there are cases in which the radiation amount detection pixels are formed with smaller sizes than the imaging pixels, as a result of which pixel values obtained from the radiation amount detection pixels are smaller and the signal-to-noise ratio is lower. Further yet, in a configuration in which charges are read out from sets of plural radiation amount detection pixels connected to the same signal line, signals from defective pixels may be obscured by signals from normal pixels.
Thus, when defects of radiation amount detection pixels are detected and pixel values thereof acquired, signal components of imaging pixels, offset components of various devices and the like are superimposed on these pixel values. Therefore, it is difficult to acquire accurate pixel values of the radiation amount detection pixels. If the radiation amount detection pixels are small in size and the pixel values thereof are small, and the detection of defects among the radiation amount detection pixels may even become more difficult.
The present invention provides a radiation imaging device that may improve the detection accuracy of defective pixels by acquiring accurate pixel values of radiation amount detection pixels, a non-transitory computer readable medium storing a program for controlling this radiation imaging device, and a pixel value acquisition method.